First Rocky World Confirmed Around Another Star

One of the smallest exoplanets yet discovered has just been
confirmed as a rocky world, scientists announced.

The planet, called CoRoT-7b, is the first planet beyond
our solar system with a proven density similar to Earth's, astronomers say.
Most known exoplanets are large gas giants like Jupiter.

"We have indications that other
exoplanets could be rocky, but it?s the first time that the density of such
a planet has been measured," said study team member Claire Moutou of the Laboratoire
d?Astrophysique de Marseille in France. "We are really sure it's
rocky."

Though its terrestrial surface renders CoRoT-7b more similar
to Earth than many other exoplanets are, it's still a far cry from a familiar
setting. The planet orbits extremely close to its star ? about 1.6 million
miles (2.5 million km), or 23 times closer than Mercury is to the sun. At this
range, the planet's surface temperatures are scorching, with highs above 2,000
degrees Fahrenheit (1,000 degrees Celsius) on the star-facing side.

CoRoT-7b's close proximity to its star means that the planet
is likely to be tidally
locked, with one side always facing its sun and the other side always in
darkness.

"Probably the day side is very hot and is pure lava,
boiling, and the other side probably is very cold, and it could be rocky with
some mountains," Moutou told SPACE.com. "It's not possible that there
is liquid water."

Crowded universe

CoRoT-7b was
discovered in February 2009 by the CoRoT space telescope, a European
collaboration. The tiny planet was discovered orbiting a star slightly smaller
and cooler than our sun, about 500 light-years away. As the planet passed in
front of its star, it eclipsed a small portion of the star's light, causing a
dip in brightness.

This dip was enough to tell that a planet existed, and to
estimate the planet's distance from its host star and its radius, which is
about 80 percent larger than Earth's. But to learn its density, which would
reveal whether it is a rocky or gas planet, requires a precise measurement of
the parent star's velocity, which is slightly warped by the planet's small
mass.

To make this measurement astronomers used the High Accuracy Radial
velocity Planet Searcher (HARPS) spectrograph on the European Southern
Observatory's 3.6-meter telescope at La Silla Observatory in Chile. The new
data revealed that CoRoT-7b has a mass about five times that of Earth, making
it one of the lightest
exoplanets yet found.

With the planet's mass and radius, the researchers
calculated its density (about 4.7 grams per cubic centimeter), which placed it
in firm rocky territory.

"This is the first proof of the detection of a rocky
planet," planet-formation theorist Alan Boss of the Carnegie Institution
of Washington told SPACE.com.

"It shows that rocky planets really are commonplace,"
said Boss, who was not involved in the new research. "The estimates are
that about 30 percent of sun-like stars have these hot and warm super-Earths,
and now that we know the density of one of them, it is easy to make the claim
that most of the rest of them are probably rocky too. The evidence is becoming
overwhelming that we live in a crowded universe."

Looking for life

Finding a rocky planet with an Earth-like density takes us
one step closer to discovering another planet similar to our own. A twin-Earth
beyond the solar system could offer the best chance of finding life elsewhere
in the universe, scientists say.

Although CoRoT-7b's lack of liquid water means it's unlikely
to host life, the planet's discovery is still a promising sign. CoRoT and
NASA's Kepler space observatory are both up there as you read this, seeking
such a discovery.

"We are searching for any kind of exoplanets,"
Moutou said. "We're trying not to be biased by our own system, but of
course we would be very interested to find a planet where life could develop.
This one is not habitable, but some future planets of this kind could allow
life to develop. This is our long-time goal, to find an analog to Earth."

The research team, led by Didier Queloz of the Geneva
Observatory in Switzerland, described the results in a paper to be published in
the Oct. 22 issue of the journal Astronomy and Astrophysics.